Doctor Who has had various companions who have chosen to travel with him for there own reason. Every Doctor has had companions, as many as 10(1st doctor, spread out through 4 season)

The Doctor’s lastest companion, Rose, is the first companion of the Doctors with a fully fleshes out life story that the audience gets to see from her debut story. She, unlike other companions, has not been cut off from her family or the world she knew.

A high-class dame of company in the fireflyversethey are a cross between the oriental-style Geisha and the classic courtesan of renaissance Italy. often recognized as the highest nobility of the 'Verse

"my cousin hopes to become a companion"
"There are advantanges to having a companion on the ship"